The production of chlorine and caustic soda is nowadays one of the most relevant electrochemical industrial processes and is carried out in plants based on three distinct technologies, namely the membrane, mercury cathode and diaphragm technologies. The membrane technology is characterised by low electrical energy consumption and by the absence of environmental issues. The two remaining, mercury cathode and diaphragm technologies, which became established during the years following World War 2, were initially characterised by high electrical energy consumption and by serious problems of environmental nature at the time the membrane plant commercialisation was taking place. Nevertheless, both technologies were able to survive, being nowadays still applied in plants whose production represents 60-70% of the world total. Such survival was permitted both for technical reasons, allowing to achieve a substantial decrease in the energy consumption and to reduce or even eliminate the environmental issues (in particular with a substantial decrease in mercury release and with the replacement of the asbestos fibres with fibres of alternative environmentally-friendly composition in diaphragm production) and for financial reasons fundamentally associated with the investment costs, evidently lower in plants already paid-back to a large extent.
As regards the reduction in energy consumption, the diaphragm technology saw the introduction of a series of innovations regarding in particular, although not exclusively, the anode nature and structure. The original anodes consisting of graphite plates were replaced by anodes formed with titanium coarse meshes, configured so as to generate a sort of flattened box (whence the term of current technical use of “box anodes”), provided with a superficial catalytic coating, for instance a ruthenium and titanium mixed oxide coating, suitable for favouring the chlorine evolution reaction. The cell voltage, although significantly decreased, was still negatively influenced by the remarkable gap, indicatively 6-8 mm, existing between the surfaces of the anodes and of the facing diaphragms. For this reason, the box anode was replaced by the expandable anodes, again characterised by a flattened box shape but with the difference that the two major surfaces, again consisting of titanium coarse mesh provided with a catalytic coating, are secured to the central current-collecting stem by elastic sheets, known in the field as “expanders”, capable of simultaneously ensuring the electric current transmission alongside a certain mobility. With this type of design, the gap between the anode and diaphragm surfaces could be reduced to about 2-3 mm, with a consequent lessening of the cell voltage and thus of the energy consumption.
Further improvements made to the expandable anode structure consist of devices directed to achieve a better circulation of the brine, with the double aim of maintaining a high chloride concentration on the surface of the catalytic coating and of quickly removing the chlorine bubbles and prevent their adhesion to the diaphragm, thereby ensuring a further cell voltage decrease. Brine circulation devices are, for instance, represented by a suitable shaping of the expanders, by flow deflectors installed on the top of the anodes, and by the substitution of the coarse mesh with vertical plates secured for example to a planar supporting sheet, with the apex of the plates maintained in any case at a distance of 1.5-3 mm from the diaphragm surface. In accordance with a similar device, the plates are fixed on the apexes of folds formed by means of a suitable shaping of the supporting sheet.
The gap between anode and diaphragm surfaces was finally eliminated with a further energy gain through the use of particular expandable anodes associated both with additional compressing elastic elements capable of safely maintaining the movable surfaces of the anodes in contact with substantially the whole diaphragm surface, and with a flattened fine mesh applied upon the previously employed coarse mesh. The fine mesh has the purpose of preventing the surface irregularities of the coarse mesh from eventually damaging the diaphragm with consequent current efficiency drop and short-circuiting hazards. The catalytic coating is applied to both meshes or preferably, in order to limit the production costs, to the fine mesh only.
Anode structures were further modified maintaining the catalytic-coated fine mesh unaltered and replacing the coarse net with horizontally or vertically arranged parallel plates having the purpose of improving the brine circulation. The hydraulic regime guaranteed by the latter expandable-type anode and the simultaneous elimination of the diaphragm-to-anode surface gap allows obtaining better cell voltages and hence a lower electrical energy consumption per unit of product chlorine, for instance 2300 kWh per tonne.
However, these expandable anodes present some inconveniences: in particular, it can be noticed that after about 1000 hours of operation the cell voltage tends to increase with a simultaneous decrease in the current efficiency accompanied by a significant increase of the oxygen content in chlorine. As a consequence, an increase in the electrical energy consumption and an intolerable diminution in the quality of the product chlorine take place. Although no certain proof exists, the cause of such a performance deterioration might be attributed to the progressive penetration of the fine mesh into the diaphragm bulk. If the above assumption is correct, the chlorine evolution takes place at least partially within the diaphragm superficial layers withholding at least a fraction of the bubbles with an electric resistance and hence a cell voltage increase. Furthermore, the alkalinity certainly present inside the diaphragm reacts with the trapped chlorine forming hypochlorite with an electrolysis efficiency drop.
The invention is directed to overcome the above described drawbacks of the prior art by means of a novel expandable anode design.